Low stratospheric temperatures are known to be responsible for heterogeneous chlorine activation that leads to polar ozone depletion. Here, we discuss the temperature threshold below which substantial chlorine activation occurs. We suggest that the onset of chlorine activation is dominated by reactions on cold binary aerosol particles, without the formation of polar stratospheric clouds (PSCs), i.e. without any significant uptake of HNO3 from the gas phase. Using reaction rates on cold binary aerosol in a model of stratospheric chemistry, a chlorine activation threshold temperature, TACL, is derived. At typical stratospheric conditions, TACL is similar in value to TNAT (within 1–2 K), the highest temperature at which nitric acid trihydrate (NAT) can exist. TNAT is still in use to parameterise the threshold temperature for the onset of chlorine activation. However, perturbations can cause TACL to differ from TNAT: TACL is dependent upon H2O and potential temperature, but unlike TNAT is not dependent upon HNO3. Furthermore, in contrast to TNAT, TACL is dependent upon the stratospheric sulfate aerosol loading and thus provides a means to estimate the impact on polar ozone of strong volcanic eruptions and some geo-engineering options, which are discussed. A parameterisation of TACL is provided here, allowing it to be calculated for low solar elevation (or high solar zenith angle) over a comprehensive range of stratospheric conditions. Considering TACL as a proxy for chlorine activation cannot replace a detailed model calculation, and polar ozone loss is influenced by other factors apart from the initial chlorine activation. However, TACL provides a more accurate description of the temperature conditions necessary for chlorine activation and ozone loss in the polar stratosphere than TNAT.

We present Polar Ozone and Aerosol Measurement III (POAM) measurements of Antarctic dehydration from 1998 to 2003 and compare these measurements with calculations performed with the Integrated Microphysics and Aerosol Chemistry on Trajectories (IMPACT) microphysical model. Previous work has shown that while dehydration is not very sensitive to reasonable changes of microphysical parameters, it is very sensitive to changes in temperature. We shall therefore compare dehydration as measured by POAM with IMPACT model runs based on four meteorological analyses: United Kingdom Meteorological Office (UKMO), European Centre for Medium‐Range Weather Forecasts (ECMWF), National Centers for Environmental Prediction Climate Prediction Center (NCEP‐CPC), and NCEP reanalysis. For the years 1998–2000, the agreement between the minimum water vapor found in the POAM measurements and that from all of the model runs is always within 0.5 ppmv. The disagreement between POAM and some of the models is larger in the years 2001–2003, growing as large as ∼1 ppmv, but the agreement between the minimum POAM water vapor and the water vapor calculated using the NCEP reanalysis is always within 0.2 ppmv. If we infer a temperature bias from the difference between the NCEP reanalysis model runs and the POAM minimum water vapor measurements, we find that this temperature bias is <0.5 K for each of the 6 years from 1998 to 2003, but it is often larger for the other meteorological analyses.

Stratospheric dehydration and high aerosol extinctions are examined for the 1998 Antarctic winter using the Integrated Microphysics and Aerosol Chemistry on Trajectories (IMPACT) model and data obtained by the Polar Ozone and Aerosol Measurement (POAM) III instrument. The model is applied to individual air parcels which are advected along 3‐D trajectories using the United Kingdom Meteorological Office (UKMO) global wind and temperature fields. Model results are compared to water vapor and aerosol extinction measurements obtained with the POAM instrument. Results suggest that the water vapor mixing ratio at the end of the season is predicted with reasonable accuracy. However, dehydration occurs more rapidly in the simulation than is indicated by the POAM data. In addition to dehydration results, the frequency of high aerosol extinction measurements is examined for all model runs and compared to POAM data. The aerosol extinction comparisons are consistent with the assumption that heterogeneous nitric acid trihydrate (NAT) freezing occurs in approximately 1% of all particles. Various model parameters influencing ice cloud microphysics are altered to examine their effects on both the water vapor mixing ratio and high aerosol extinction events. While a reduction in the ice accommodation coefficient and an increase in the ice nucleation barrier both improve the agreement in the water vapor mixing ratio, the agreement in aerosol extinction is worsened. Extinction comparisons suggest that the model results are consistent with either high or low NAT‐ice lattice compatibility factors, although intermediate values agree poorly with POAM data. The extent of dehydration is highly dependent on temperature; therefore, an uncertainty as small as ±1 K in the UKMO temperature fields may significantly change the model results.

The Integrated Microphysics and Aerosol Chemistry on Trajectories (IMPACT) model is used to study polar stratospheric cloud (PSC) formation and evolution in the Antarctic vortex. The model is applied to individual air parcel trajectories driven by UK Met Office (UKMO) wind and temperature fields. The IMPACT model calculates the parcel microphysics, including the formation and sedimentation of ice, nitric acid trihydrate (NAT), sulfuric acid tetrahydrate (SAT), and supercooled ternary solution (STS) aerosols. Model results are validated by comparison with data obtained by the Polar Ozone and Aerosol Measurement (POAM) III solar occultation instrument and are examined for 6 years of POAM data (1998–2003). Comparisons of POAM water vapor and aerosol extinction measurements to the model results help to constrain three microphysical parameters influencing the formation and growth of both type I and type II PSCs. Principally, measurements of aerosol extinction prove to be valuable in differentiating model runs; the relationship of aerosol extinction to temperature is determined by the various particle types as they form and grow. Comparison of IMPACT calculations of this relationship to POAM measurements suggests that the initial fraction of nuclei available for heterogeneous NAT freezing is approximately 0.02% of all aerosols. Constraints are also placed on the accommodation coefficient of ice and the NAT‐ice lattice compatibility. However, these two parameters have similar effects on the extinction‐temperature relationship, and thus a range of values are permissible for each.

The spectral dependence of aerosol extinction coefficient (AEC) for several hundred of thousands of stratospheric aerosol and PSC realizations has been calculated based on Mie-theory algorithms. The means and covariance matrices for different scenarios of PSC transformation have been constructed. It has been shown that it is possible to use only 4 eigenvectors of AEC covariance matrices for approximation of AEC spectral dependence with 5% accuracy in 0.29 to 1.56 μm spectral region. The possibilities of retrieving the size distribution function (SDF) and its moment from AEC measurements with 5–25% accuracy have been studied. The regression method makes it possible to decrease essentially the relative a priori uncertainty of SDF of combined ensemble for the size range of 0.06–2 μm. This decrease is maximal for the 0.5–0.7-μm size particles (from 400 to 60%). Absolute a priori uncertainty of SDF decreases (from 3.5 to 0.35 cm) in the size range of about 0.3 μm. In the range of the uncertainty curve maximum (0.1 μm), the absolute a priori uncertainty decreases from 7.6 to 3.8 cm at the 5% AEC error, and to 5.5 cm at 25%. Total cross-section area S is the best-determined value from all SDF moments. For combined ensemble, a priori uncertainty of S decreases by 4–5.5 times.

Microphysical properties of stratospheric aerosol and polar stratospheric clouds (PSCs) are simulated based on the up-to-date numerical model of formation and transformation of aerosol particles. Overall, the analyzed ensemble totals 255949 realizations and may be considered as a broad representative ensemble of stratospheric aerosol states and PSC under polar and midlatitude winter conditions of the northern hemisphere (45–90°N). The means and covariance matrices of the size distribution function (SDF) for different scenarios of PSC transformation are constructed. Possibilities of optimally parameterization of SDF and its higher-order moments are explored. It is shown that, to describe all SDF realizations with the error no larger than 5–10%, it is sufficient to use just 6 expansion coefficients, rather than specifying SDF a priori in 39 bins. Use of “foreign” vectors as a basis in parameterizations by use of other models insignificantly changes the relative errors of SDF parameterization.

Algorithms to simulate the statistical microphysical and optical models for aerosol and polar stratospheric cloud (PSC) are described. Examples of such models for stratospheric and tropospheric aerosols and PSC are given. Different ways of applying the statistical aerosol and cloud models are discussed:optimal parameterization of spectral dependences of aerosol extinction coefficient using the natural orthogonal basis;multiple regression for estimating the optical parameter from measured one (for example, estimation of scattering coefficients from SAGE III multiwavelength measurements of aerosol extinction coefficients);retrieval of microphysical properties of stratospheric aerosol and PSC from SAGE III extinction measurements;lidar sounding.

We observed a plume of air highly enriched in carbon monoxide and particles in the stratosphere at altitudes up to 15.8 km. It can be unambiguously attributed to North American forest fires. This plume demonstrates an extra‐tropical direct transport path from the planetary boundary layer several kilometers deep into the stratosphere, which is not fully captured by large‐scale atmospheric transport models. This process indicates that the stratospheric ozone layer could be sensitive to changes in forest burning associated with climatic warming.

A number of recently published papers suggest that mountain‐wave activity in the stratosphere, producing ice particles when temperatures drop below the ice frost point, may be the primary source of large NAT particles. In this paper we use measurements from the Advanced Very High Resolution Radiometer (AVHRR) instruments on board the National Oceanic and Atmospheric Administration (NOAA) polar‐orbiting satellites to map out regions of ice clouds produced by stratospheric mountain‐wave activity inside the Arctic vortex. Lidar observations from three DC‐8 flights in early December 1999 show the presence of solid nitric acid (Type Ia or NAT) polar stratospheric clouds (PSCs). By using back trajectories and superimposing the position maps on the AVHRR cloud imagery products, we show that these observed NAT clouds could not have originated at locations of high‐amplitude mountain‐wave activity. We also show that mountain‐wave PSC climatology data and Mountain Wave Forecast Model 2.0 (MWFM‐2) raw hemispheric ray and grid box averaged hemispheric wave temperature amplitude hindcast data from the same time period are in agreement with the AVHRR data. Our results show that ice cloud formation in mountain waves cannot explain how at least three large‐scale NAT clouds were formed in the stratosphere in early December 1999.

POAM solar occultation observations from 1994 to present are studied for the purpose of determining Type I PSC formation characteristics and winter-long evolution. This study examines PSC observations from many years on a common basis to see if characteristics can be identified. The results show that Type Ia PSCs form at the beginning of the winter, within several days of the first drop in temperature below T_NAT, and peak early in the winter. Type Ia PSCs typically out number Ib PSCs over the winter, especially at the beginning of the winter. Type Ia and Ib PSC observations continue throughout the winter. Micro-physical models of PSC formation must match these observed characteristics. Some models predict that temperatures must be more than 5 K below T_NAT for five days before significant freezing can occur. This is not seen in the POAM observations. Differences in PSC characteristics between the first two Arctic winters (1994-1995 and 1995-1996) and later winters also suggest the influence of volcanic perturbations on PSC formation. Type Ia and Ib PSC Characteristics observed by POAM III and SAGE III for the 2002-2003 Arctic winter are compared.

Simulations of the 1999–2000 winter have tested the effect on polar stratospheric clouds (PSCs) of the homogeneous freezing of liquid ternary solutions into nitric acid trihydrate (NAT) and nitric acid dihydrate (NAD). Proposed laboratory‐derived volume‐based and surface‐based homogeneous freezing rates have both been examined, including different assumptions about the extrapolation of laboratory measurements to atmospheric conditions. Widespread PSC formation and denitrification are possible in several of the scenarios examined. However, the simulations are all unable to explain the solid‐phase PSCs observed early in the 1999–2000 winter and are unable to reproduce the measured extent of vortex denitrification. These problems can both be attributed to the relatively cold temperatures, more than 5 K below the NAT condensation point, necessary for homogeneous freezing to be effective at producing solid‐phase PSCs. Therefore synoptic‐scale homogeneous freezing appears unlikely to be the primary mechanism responsible for solid‐phase PSC formation.